1. Field of the Invention
The present invention relates to an eye refractive power measurement apparatus for measuring eye refractive power of an eye of an examinee objectively.
2. Description of Related Art
Conventionally, there is known an eye refractive power measurement apparatus having a measurement optical system which includes a projection optical system for projecting measurement light from a measurement target onto a fundus of an eye of an examinee via an objective lens and a photo-receiving optical system for photo-receiving the measurement light reflected from the fundus via the objective lens in the projection optical system by using a photodetector. In such a configuration, the objective lens is shared by the projection optical system and the photo-receiving optical system; therefore, there is such a problem that the measurement light from the measurement target is reflected by the objective lens to inadvertently enter the photodetector as noise light, resulting in a decrease in measurement accuracy. Accordingly, proposed as a solution to this problem is an eye refractive power measurement apparatus in which the measurement target is arranged at a decentered position from an optical axis of the objective lens to dissipate the measurement light reflected by the objective lens to be off-axis, preventing the measurement light from entering the photodetector as noise light (see Japanese Patent Application Unexamined Publication No. Hei 4-73038).
Additionally, as for the measurement optical system of the eye refractive power measurement apparatus, in a case where the position of the measurement target is fixed, an image of the measurement target formed on the fundus is clear if the examinee's eye is close to emmetropia; however, if the examinee's eye has a refractive error, the image of the measurement target formed on the fundus becomes blurred and an S/N ratio (signal-to-noise ratio) at the photodetector is lowered to decrease measurement accuracy. In addition, in a case where the position of the photodetector is fixed, not only the image of the measurement target formed on the photodetector becomes blurred, but also its position on the photodetector changes if the examinee's eye has a refractive error; therefore, a resolution is lowered to decrease measurement accuracy if a wide measurement range is to be covered. This problem can be overcome in such a manner that the measurement target and at least one of the photodetector and an image-forming member in the photo-receiving optical system are moved in a direction of their optical axis to have a positional relationship optically conjugate with the fundus.
However, a combination of the configuration that the measurement light reflected by the objective lens is dissipated to be off-axis and the configuration that the measurement target and at least one of the photodetector and the image-forming member are moved in the optical axis direction brings about change in a reflection angle of the measurement light reflected by the objective lens; therefore, it is difficult to prevent noise light from entering the photodetector over a wide measurement range.